Ink jet printing is a non-impact printing method that produces droplets of ink that are deposited on a print substrate such as paper or transparent film in response to an electronic digital data signal. Thermal or bubble jet drop-on-demand ink jet printers have found broad applications as output for personal computers in the office and in the home.
In existing thermal ink jet printing processes, the printhead typically comprises one or more ink jet ejectors, as disclosed in U.S. Pat. Nos. 4,601,777, 4,532,530, 4,412,224, 4,410,899, and 4,251,824. Each ejector includes a channel communicating with an ink supply chamber, or manifold, at one end and an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels at a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink within the respective channel to form a bubble that expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. This is a very temporary phenomenon, and the ink is quickly propelled toward a print substrate. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation from the nozzle of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity for propelling the droplet in a substantially straight line direction towards a print substrate, such as a piece of paper. Important properties of the ink in this context include the ink's viscosity and surface tension. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink jet printing is known as "drop-on-demand" printing.
Another type of drop-on-demand ink jet printing is called piezoelectric ink jet printing. This ink jet printing system has an ink filled channel with a nozzle on one end and a regulated piezoelectric transducer near the other end to produce pressure pulses according to the digital data signal.
A third type of drop-on-demand ink jet printing is called acoustic ink jet printing which can be operated at high frequency and high resolution. The ink jet printing system utilizes a focused acoustic beam formed with a spherical lens illuminated by a plane wave of sound created by a piezoelectric transducer. The focused beam reflected from a surface exerts a pressure on the surface of the liquid ink, resulting in ejection of small droplets of ink onto an imaging substrate. Aqueous ink jet inks can be used in this printing system.
An ink jet printing method that is different from the drop-on-demand ink jet printing is called a continuous ink jet printing. In this ink jet printing system, ink is emitted from a nozzle in a continuous stream under pressure. The stream is ejected out of orifice and perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break up point, the electrically charged ink droplets are passed through an applied electrode which is controlled and switched on and off according to the digital data signals. The charged ink droplets are passed through a controllable electric field which adjusts the trajectory of each ink droplet in order to direct it to either a gutter for ink deletion and recirculation or a specific location on a print substrate to create image. Multiple orifices or nozzles can be used to increase imaging speed and throughput.
In a drop-on-demand ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print substrate, either by moving the print substrate relative to a stationary printhead, or vice-versa, or both. In some types of apparatus, a relatively small printhead moves across a print substrate numerous times in swaths (i.e., multiple passes) to print a desired image. In this case, a desired image including a color image is completely produced on a print substrate in several swaths before the substrate is advanced. This type of printing is called multi-pass or checkerboard ink jet printing process. In checkerboard ink jet printing (or multiple pass method), the printhead passes over the print substrate and provides ink at a desired locations (e.g., printing only even or odd numbered dots in a swath). On one or more subsequent passes the remaining dots in the image are printed before the print substrate is advanced. This type of multi-color ink jet printing is commonly found in a desk-top ink jet printer including a thermal ink jet printer. It produces good color images on plain paper, but at a slower printing speed.
Alternatively, a stationary ink jet printhead that consists of an array of ejectors and extends the full width of a print substrate may pass ink down the print substrate to give full-page images, in what is known as a "full-width array" ink jet printer. When the printhead and the print substrate are moved relative to each other, image-wise digital data is used to selectively activate the thermal energy generators (e.g., resistors) in the printhead over time so that the desired image can be created on the print substrate quickly in a single pass mode. The full-width-array printhead is generally preferred to be in a stationary position while the print substrate is continuously moving to receive ink image as it passes through the printhead. However, the full-width array printhead can also be moved across the print substrate if it is desired. In a multi-color ink jet printing process, several "full-width array printheads" including cyan, magenta, yellow, and black printheads and their corresponding ink jet inks are used to provide different color images on the print substrate. Fast ink jet printing can be achieved by using the full-width array printheads.
With the demand for higher resolution printers, the nozzles of a printhead or full-width array printhead in ink jet printers are correspondingly decreasing in size. Nozzle openings of a printhead are typically about 50 to 80 micrometers (or microns) in width or diameter for 300 spots per inch (spi) resolution printers. With the advent of higher resolution (e.g., 400 spi and 600 spi) printers, these nozzle openings are smaller and are typically about 10 to about 49 micrometers (microns) in width or diameter. These printheads and full-width array printheads with small nozzle dimensions require special inks that do not easily clog the small nozzle openings.
A major concern with all ink jet printers, and high resolution ink jet printers in particular, is clogging of nozzles during operation and between operations. This is caused by evaporation of an organic solvent or water from the opening of the nozzle. In dye-based inks, this can cause crystallization or precipitation of soluble components such as dye or solid additives as well as viscosity increase of ink fluid. In pigment-based inks, this evaporation can cause precipitation of the pigment particles due to flocculation or aggregation, or precipitation of solid additives as well as viscosity increase of the ink fluid. Initial evaporation of water and solvent generally causes an increase in ink viscosity, which affects the ability of the heater (i.e., a resistor) of a printhead to fire a drop of ink properly through a nozzle.
The inception of clogging may cause distortion of the image or alphanumeric characters being printed by the printhead. This may appear as a drop of ink that is displaced from its intended position. Sometimes two drops of ink will be formed equally spaced from the intended target position. Sometimes small numerous satellite drops are produced. On some occasions the drop may even reach its intended position but at a lower drop volume or drop mass producing a lower optical density image. Ultimately, the clogged nozzle may fail to fire entirely, and no image can be generated on a print substrate.
Ink jet printers are designed to prevent excessive evaporation of solvent from printhead nozzles by sealing the printhead or printbar (comprising many butted printheads such as a partial width printhead and a full-width array printhead) in an air-tight chamber when not in use. These devices can become ineffective with continued printer use because dried ink deposits can be formed at the front face of a printhead due to ink flooding or at the rubber seals of the air-tight chamber, causing the system to lose its air-tight condition. Another device used to prevent clogging of the printhead nozzle is a wiper that removes solid formed near or at the opening of a nozzle. This device in some cases may become ineffective because of the depth of the plug or because of sufficient hardness of the plug, which thereby resists mechanical removal. Another clogging remedy is the use of vacuum suction to clear the nozzle of any deposits. These devices are effective only for soft ink clogging. They are usually inefficient to remove hard ink deposits, and add considerable expense to the costs of the printer.
Another commonly used mechanism to cure clogging is to clear the nozzle by firing the printhead in a non-image mode, e.g. into a collection receptacle. While this solution is an effective remedy, it requires that the ink forms a soft or non-cohesive plug. To make this non-image clearance process effective, the ink in the nozzle must be mechanically or cohesively weak for easy jetting or ink removal. Frequent firing of an ink jet nozzle for maintenance purposes may be needed in order to avoid the formation of hard solid plug.
Therefore, a critical requirement for an ink jet ink is the ability of the ink to remain in a fluid and jettable condition in a printhead opening that is exposed to air. The maximum idling time that still allows a printhead to function properly with a transit time .ltoreq.100 microseconds for an ink to travel a distance of 0.5 mm after a period of non-use or idling is called the latency or decapped time. This test is run with the printhead or nozzles uncovered or decapped and generally at a relative humidity of 15%. The time interval is the longest length of time that the printhead, uncovered, will still fire a specified drop without a failure. The longer the latency time rating, the more desirable is the ink for use in an ink jet printer. U.S. Pat. No. 4,840,674 to Schwarz, the entire disclosure of which is incorporated herein by reference, describes an ink jet ink having sulfone derivatives in combination with dyes and other ink additives. However, these sulfone derivatives were not used in pigment inks and they were not disclosed to have desirable anti-curl properties which are desired for ink jet printing with archival capability. Thus, there is a need to improve ink latency (particularly for high resolution printheads) and anti-curl property by using a novel water soluble or miscible humectant or co-solvent in inks.
Another important requirement for ink jet inks, especially for pigment-based inks, is for the pigment particles to remain stable and uniformly dispersed in the ink throughout the life of the ink jet cartridge. Dye-based ink jet inks suffer from deficiencies in waterfastness (water resistance) and lightfastness (light resistance) after being printed on various substrates. Pigments provide an image, on a wide variety of substrates, having high optical density and sharp edges with very good waterfastness and lightfastness. Therefore, pigments are a preferred alternative to dyes in an ink jet ink, provided that the pigment particles and dispersions can be made stable to prevent undesired flocculation and/or aggregation and settling. U.S. Pat. No. 5,281,261 to Lin, and pending U.S. patent application Ser. No. 08/810,841 to Lin et. al., the entire disclosures of which are incorporated herein by reference, describe an ink jet ink having pigment particles in inks and in combination with dyes and other ink additives.
Great effort has been made in attempts to provide both dye-based and pigment-based ink jet inks with acceptable latency and stability for high speed and high resolution ink jet printing. However, there continues to have a demand for inks having many above-mentioned desirable characteristics.
Moreover, certain ink jet printers require ink jet inks to have sufficient optical density in a single pass method, i.e. without applying additional ink to the substrate or paper. Additionally, certain ink jet printheads in printers are designed to provide enhanced resolution such as, for example, a color ink jet printer capable of providing genuine 600 spi (printhead resolution) color ink jet printing as compared to the currently used 360 spi and 300 spi printers in the state-of-the-art commercial products. These devices (printheads or full-width array printheads or printbars) require specially refined inks that do not easily cause clogging or plugging of the ink jet nozzles that, as mentioned above, are significantly narrower than those of 360 spi and 300 spi printers. Many current state-of-the-art color printhead nozzles for commercial color thermal ink jet printers are limited to a resolution of equal to or less than 360 spi.
Many state-of-the-art commercial ink jet inks, including dye-based inks and pigment-based inks, show a short latency (about 10 sec.) when they are used in conjunction with a high resolution (&gt;360 spi, for example 600 spi) printhead with a channel width or nozzle diameter of about 10 to 49 microns. Accordingly, such inks are not suitable for high resolution color ink jet printers because they have undesired jetting characteristics and large unstable pigment particles (&gt;3 microns with agglomeration or flocculation) that can easily cause clogging of printhead nozzles. Thus, there is a need to provide dye-based and pigment-based inks that have good latency, especially when they are used in the aforementioned high resolution printheads.
There is also a need in the art for developing new aqueous ink compositions comprising a colorant of a dye or pigment and other ink additives that can be utilized in high resolution ink jet printers. There is also a need for inks that provide high optical density not only for printing in a single application or pass (for high speed printing) method but also in multiple passes (multi-pass) method. Furthermore, there is a need to provide inks that are capable of printing at high speed. This requires a high jetting frequency response (e.g., greater than 3.0 KHz, and preferably greater than 7.0 KHz).
Aqueous inks used in ink jet printing generally have water as a major component. Water has the advantage of being non-toxic, non-combustible and environmentally sound relative to non-aqueous inks, which are largely composed of organic solvents. Water is also an excellent medium for dispersing pigments or dissolving dyes.
The use of water in large concentrations, however, also has several disadvantages, as disclosed in U.S. Pat. No. 5,356,464 to Hickman et al. Water has a fast evaporation rate relative to high-boiling organic solvents, which reduces the ink latency. Water also interacts with paper to cause two major distortions known as paper cockle and paper curl. Paper cockle is a distortion in which bumps, indentations and other irregularities are randomly produced on the printed paper, giving the paper a "wrinkled" or "wavy" appearance. Paper cockle can cause the paper in the deformed area to rub against the printhead in the printing process. Paper curl (or curl) is a phenomenon in which the edges of the paper migrate towards the center of the paper after the printing and aging. In extreme cases, curl causes the paper to assume the shape of a scroll upon aging. The curl direction may be on the printed side of the paper, or it may be on the non-printed side (the latter being known as "reverse curl"). U.S. Pat. No. 5,356,464 discloses aqueous ink compositions including anti-curl agents such as 1,3-diols, 1,3,5-triols, amino-1,3-diols and their polyoxyalkylene derivatives. Some of those materials have high viscosity at room temperature and cause poor jetting performance or jetting failure at a concentration in ink that is required to effectively reduce paper curl. Thus, there is a need for the development of anti-curl agents which give low ink viscosity to avoid poor jetting performance.
The use of heating elements (commonly employed to increase the rate of drying of aqueous inks) may be insufficient to reduce paper curl. Various mechanical devices to reduce curl such as heated rollers and tension applicators have been tried. These devices are only marginally effective and add considerably to the cost and size of the printer. Heated rollers used to reduce curl differ from the heaters used to increase drying rate, such as microwave heating, radiant heating, heated plate, heated drum, forced hot air heating, convection heating, and the like. In heaters to reduce curl, heat is typically applied to both sides of the paper after printing and can potentially cause ink smearing; in heaters to increase the drying rate, heat may be applied at any desired stage of printing including before, during, and after the printing process. Microwave dryers, for example, are set forth in U.S. Pat. Nos. 5,220,346 to Carreira et al. and 4,327,174 to Von Meer, the disclosures of which are incorporated herein by reference. The inks employed in ink jet printers having a microwave dryer comprise salts including mono-valent and multi-valent metal salts that improve the rate of drying. Such additives, however, do not reduce paper curl.
To reduce cockle and curl in ink jet printers, efforts have been made to provide anti-curl and anti-cockle agents to reduce this problem. For example, U.S. Pat. No. 5,356,464 to Hickman et al. and U.S. Pat. No. 5,169,437 to You, the entire disclosures of which are incorporated herein by reference, describe anti-curl agents that may be utilized in ink jet inks. U.S. Pat. No. 5,207,824 to Moffatt et al. describes an ink jet ink comprising an anti-cockle agent for thermal ink jet printers. As mentioned above, some of the anti-cockle and anti-curl agents, especially the triols, are very viscous and can significantly increase ink viscosity, thereby causing poor jetting performance and jetting failure especially in a high resolution printhead with narrow nozzles.
Thus, there is a need for aqueous ink compositions that can reduce paper curl, thus eliminating the need for using expensive, ineffective and cumbersome mechanical devices or special print media. There is also a need to have an ink component that can be used in conjunction with known anti-cockle and anti-curl agents to reduce ink viscosity and to provide good jetting property. In addition, there is also a need for aqueous ink compositions that have low viscosity and can be used either with or without heaters in the printing process to increase ink penetration rate and image drying.
Another common problem encountered in employing aqueous ink jet ink compositions is kogation. Occasionally, as ink in an ink jet printhead is heated and vaporized, some ink component will undergo thermal breakdown. This decomposition leads to residue deposition on the resistor's surface in a process known in the art as "kogation." Such deposits prevents effective heat transfer from the heater to the ink on the heater (resistor) surface, thereby causing reduced bubble formation, decreased ejection velocity of the ink drops, and reduced ink drop volume delivered to the print substrate. Consequently, print quality is reduced and failure in bubble formation may result in failure of the ink jet printer to print. Thus, desirable humectants used in ink jet ink compositions must have the capability of not only reducing the rate of ink evaporation to avoid crusting and clogging of a printhead but also preventing the formation of undesired kogation.
Many known humectants employed in ink jet inks are diol derivatives such as ethylene glycol, propylene glycol, and the like. However, these diols do not have good anti-curl properties. An effective humectant and anti-curl agent should have a good water solubility and low vapor pressure. There is a need to have an ink component which can serve as humectant and anti-curl agent. Furthermore, there is also a need to have a novel humectant that can be used in conjunction with other known anti-curl agents in inks to reduce ink evaporation rate for long latency and to lower ink viscosity for achieving good jetting performance.